1. Field of the Invention
The present invention relates to cordless telephone apparatuses which includes a single base unit connected to a telephone subscriber line, and a plurality of radio telephone sets which are coupled locally by a radio circuit to the base unit, and methods of controlling the same and, more particularly, to apparatus and methods of controlling the same, which is adapted for transfer responding to a request of the active communication with the remote party (or subscriber) from the currently active radio telephone set to another one of the local radio telephone sets.
2. Description of the Related Art
In such cordless telephone apparatuses, when a first of the plurality of radio telephone sets in the apparatus is in telephone communication with a remote party through the telephone network, and the telephone communication needs to be transferred to a second one of the plurality of local radio telephone sets, in order to keep the active DC (Direct Current) loop from the network closed to maintain the network connection with the remote party, the first radio telephone set which transfers the telephone communication first performs a predetermined hold operation. The base unit holds the DC loop closed and sends a predetermined hold signal or the like indicative of that fact to the telephone set of the remote party.
Subsequently, the transferring first radio telephone set dials an extension number of the second radio telephone set which is to receive the transferred telephone communication and then, assuming that the second set goes off-hook, the first set ends its off-hook condition, that is, goes on-hook so that it no longer is in communication with the remote party.
In more detail, when the base unit receives a signal indicative of the dialed extension number of the receiving second radio telephone set from the transferring first radio telephone set, it establishes a radio circuit between the base unit and the receiving second radio telephone set and sends a ringing signal to the receiving second radio telephone set.
A bell rings at the receiving second radio set in response to the ringing signal. When the receiving second radio telephone set goes off-hook in response to the bell ringing, the base unit stops sending the ringing signal and establishes a communication channel between the receiving second radio telephone set and the office line. Then, the transfer of the telephone communication from the transferring first to the receiving second radio telephone set ends and the receiving second radio telephone set can execute a telephone communication with the outside-line subscriber at the other end of the office line.
However, no one may be available at the receiving radio telephone set to answer. It is assumed that the user at the transferring radio telephone set transfers the telephone communication to the receiving radio telephone set without knowing that no one is at the receiving radio telephone set. In that case, no one responds to the ringing tone generated by the receiving radio telephone set due to the bell ringing signal from the base unit and as a result the outside-line subscriber is kept on hold so long as the ringing is not responded to. The user at the transferring radio telephone set cannot at all know what the result of the transfer is because the transferring radio telephone set has already moved to a standby state. Therefore, there is a problem that each time the transfer occurs, it is necessary to go and see whether anybody is at the receiving radio telephone set for answer and whether the transfer has been achieved.
If there is no one at the receiving telephone set when the ringing tone is generated, there is a problem that the telephone communication cannot be retransferred to the transferring telephone set.
In order to solve these problems, an arrangement can be conceived by providing an additional pair of transferring and receiving radio telephone sets to enable establishing two radio circuits. However, this would increase the cost greatly and also the size of the base unit undesirably.
Therefore, it is an object of the present invention to provide a cordless telephone apparatus and a method of controlling same which eliminate the above problems, and are capable of ascertaining a transferred state and, if necessary, retransfer the telephone communication to the transferring radio telephone set.